Centers for Disease Control and Prevention (CDC) Director Tom Frieden announced Thursday a travel alert advising against non-essential travel to Guinea, Sierra Leone and Liberia because of the Ebola outbreak. / MANDEL NGAN, AFP/Getty Images

by Liz Szabo, USA TODAY

by Liz Szabo, USA TODAY

U.S. health officials issued a travel advisory Thursday, urging Americans to avoid non-essential travel to the three countries in West Africa dealing with the Ebola outbreak.

"Ebola is worsening in West Africa," said Thomas Frieden, director of the Centers for Disease Control and Prevention, announcing the level 3 advisory -- its strongest level -- against travel to Liberia, Guinea and Sierra Leone. Frieden said the CDC is sending 50 additional staff to West Africa to advise countries on controlling the disease. "It's the largest, most complex (Ebola) outbreak that we know of in history."

Although controlling the epidemic could take six months, the "CDC, along with others, are surging to begin to turn the tide," Frieden said. "It's not going to be quick and it's not going to be easy, but we know what to do."

Frieden said the CDC is advising against travel both for the benefit of travelers and host countries. People who are injured in a car accident and need hospital care, for example, could further stress medical staff who are struggling to cope with Ebola, and also put themselves at risk of contracting the illness.

The World Health Organization announced Thursday that the disease has killed 729 people and affected more than 1,300.

There are no effective treatments or vaccines, so the best way to control the disease will be with traditional public health measures, such as identifying those who are sick, meticulously tracking down everyone they could have exposed, monitoring those people during a 21-day "fever watch" to spot early symptoms, then isolating people until they are no longer contagious, Frieden said.

Frieden says CDC officials have worked closely with Uganda and other countries in the past, educating traditional village healers to recognize the symptoms of Ebola and training hospital staff how to identify and isolate sick patients to stop the "chain of transmission."The current outbreak is especially challenging, however, because it's occurring in countries that have never before had to grapple with Ebola, Frieden says. Some communities have become hostile and fearful of doctors, as well. In April, one month after the current outbreak began, an angry crowd attacked a medical center in Guinea where doctors were treating patients.

The National Institutes of Health plans to launch a small, early-phase trial of an experimental Ebola vaccine in September. Even in the best-case scenario, the NIH's experimental vaccine wouldn't get to workers in the field for a year, Frieden said.

"We know how Ebola spreads," Frieden said. "We know how to stop the spread of Ebola. ... It's possible to control Ebola with our current tools."

The CDC's epidemic intelligence service, known as disease detectives, will train others in West Africa to track down every contact of patients diagnosed with Ebola, Frieden says."It's like fighting a forest fire: if you leave behind even one burning ember, it can reignite the fire," Frieden said.Still, he said, "I'm confident that as we make progress over coming weeks and months, we will not only begin to tamp down these outbreaks, but also leave behind stronger system to stop Ebola. That is because countries will have emergency management systems and lab networks and trained disease detection."